feat(admin): allow editing boxes and deleting individual files

Introduce new admin capabilities to manage uploaded boxes and files:
- Add routes and handlers for editing boxes and deleting individual files.
- Implement `RemoveFileFromBox` in `UploadService` to delete a file's stored objects and remove it from the box (deleting the box if empty).
- Implement `AdminUpdateBox` in `UploadService` to update expiry, download limits, and clear password protection.
- Remove the unused `AdminFiles` handler.
- Add `.claude` to `.gitignore`.

.gitignore

@@ -15,3 +15,5 @@ backend/static/uploads/*
 .prod.env
 scripts/env/dev.env
 docker-compose.yml
+
+.claude